Patients with posttraumatic stress disorder (PTSD) have persistent fear despite the presence of safety signals. Although the inability to inhibit fear is a central problem in PTSD, it has not been well studied. One of the goals of the present study is to assess fear potentiation and fear inhibition as independent processes in humans. This study will use fear potentiation of the acoustic startle response as a measure of fear, given that it has been observed across many different species and its neural correlates are well known. The first part of the study will translate a conditional discrimination procedure (AX+/BX-), validated in animals, in order to quantify fear inhibition in PTSD. Furthermore, we will test whether the subjects are cognitively aware of the contingencies between the experimental stimuli. Previous research has found that contingency awareness plays a key factor in developing fear-potentiated startle. The present study will investigate how awareness interacts with startle in PTSD subjects. Finally, the present study will examine the relationship between contingency awareness and fear inhibition.